How Do I Pass Data Out Of The Ondatachange Method?

I have a boolean set up to check for duplicate usernames in an app. I want the boolean to return data based on the result of an onDataChange in a ValueEventListener. Here's what I

Solution 1:

If you try to do something like this, you will always be returning false, since Firebase is asynchronous, you will need to wait a little bit depending on your connection in order to return any value from dataSnapshot()

privatebooleanisUsernameValid(final String username) {

        boolean isUsernameValid;
        mReference = FirebaseDatabase.getInstance().getReference();
        Query query = mReference.child("users").child(username);
        ValueEventListener mListener = newValueEventListener() {
            @OverridepublicvoidonDataChange(@NonNull DataSnapshot dataSnapshot) {
                if (dataSnapshot.exists()) {

                    //create condition that would make isUsernameValid return false
                     isUsernameValid = true;
                }
            }

            @OverridepublicvoidonCancelled(@NonNull DatabaseError databaseError) {

            }
        };

        mReference.addListenerForSingleValueEvent(mListener);

        return isUsernameValid //if that condition is not met;
    }

To solve this issue you can create an interface that will fires right when we know we have the results from onDataChange() and then you can return anything from there

first create an interface

publicinterfaceFirebaseSuccessListener {

    voidonDataFound(boolean isDataFetched);

}

Then we just do your method that checks for the user if exists

privatevoidisUsernameValid(final String username, FirebaseSuccessListener dataFetched) {


            mReference = FirebaseDatabase.getInstance().getReference();
            Query query = mReference.child("users").child(username);
            ValueEventListener mListener = newValueEventListener() {
                @OverridepublicvoidonDataChange(@NonNull DataSnapshot dataSnapshot) {
                    if (dataSnapshot.exists()) {

                        //here we are telling to our interface that the data has been found and then we can do anything with it outside this method
                         dataFetched.onDataFound(true);
                    }
                }

                @OverridepublicvoidonCancelled(@NonNull DatabaseError databaseError) {

                }
            };

            mReference.addListenerForSingleValueEvent(mListener);


        }

And then we can call the method in order to get your value from inside onDataChange()

isUsernameValid.(newFirebaseSuccessListener() {
    @OverridepublicvoidonCallback(boolean isDataFetched) {
        if(isDataFetched){
           //you know the value is true, here you can update or request any change for example you can do this userIsOnFirebase(true);

      }else{
      userIsOnFirebase(false);
      }
    }
});


privatebooleanuserIsOnFirebase(boolean isOnFirebase){
return isOnFirebase;
}

then use this method above to return if the user is or not in your db. Make sure when you call userIsOnFirebase since it will return false if the data is still not fetched